The present invention relates to an optical pickup to be used in information recording/reproducing apparatus such as an optical disc apparatus for optically recording and reproducing information.
Recently, improvement for the optical disc apparatus is being made increasingly for size-reduction and weight-reduction purposes. The size-reduction and weight-reduction also allow improvement of its performance such as shortening the access time. For achieving these purposes, one known technique involves constructing the optical pickup with a hologram optical device. FIG. 14 shows an arrangement of a conventional optical pickup. In FIG. 14, numeral 231 designates a hologram optical device comprising a glass-made base, a hologram 235 provided on the upper surface of the base and a grating 236 provided on the lower surface of the base. As illustrated in FIG. 14, a light emitted from a semiconductor laser 230 is divided by the grating 236 into three beams, i.e., two tracking detecting sub-beams and a signal reading main beam, and directed toward the hologram 235. The light beams passing through the hologram 235 are converted into parallel rays through a collimator lens 232 and then condensed through an objective lens 233 on a disc 237 so as to be reflected thereon. The reflected light including information recorded on the disc 237 again passes through the objective lens 233 and the collimator lens 232 before diffracted by the hologram 235 to be led to a 5-divided photodetector 234. Here, the hologram 235 has 2 areas which are different in grating period from each other, and hence the incident light is wave-front-converted by the hologram 235 so as to be focused (condensed) on a predetermined position of the 5-divided photodetector 234. When a focussing error occurs, the focusing position of the 5-divided photodetector 234 varies, thereby obtaining a focusing error signal. In addition, in response to occurrence of a tracking error, a difference in intensity between the reflected lights of the 2 tracking detecting sub-beams occurs, thereby obtaining a tracking error signal.
There is a problem which arises with such an arrangement of the conventional optical pickup, however, in that the optical length from the semiconductor laser 230 to the objective lens 233 becomes long to make it difficult to reduce the thickness of the optical pickup.